1. Field of the Invention
The present invention relates to liquid phase growth methods and liquid phase growth apparatus and, more particularly, is suitably applicable to liquid phase growth methods and liquid phase growth apparatus of an immersion type in which a substrate of wafer size is held by a jig and immersed into a solution containing a growth material.
2. Related Background Art
Global environments are becoming worse because of emission of earth-warming gases such as carbon dioxide, nitrogen oxides, etc. from combustion of petroleum in thermal power generation, combustion of gasoline in automotive engines, and so on. In addition, there is future concern about a drain of crude oil and attention is thus being drawn toward power generation with solar cells as clean energy sources.
Since thin-film crystal silicon (Si) solar cells have a thin power-generating layer and are made using a small amount of the source material of Si, there is a perspective of cost reduction of the thin-film crystal Si solar cells. Since the power-generating layer is made of crystal Si, higher conversion efficiency and less deterioration can be expected as compared with solar cells of amorphous Si and others. Further, since the thin-film crystal Si solar cells can be bent to some extent, they can be used in a bonded state to curved surfaces of automotive bodies, household electrical appliances, roof tiles, and so on.
For substantiating the thin-film crystal Si solar cells, Japanese Patent Application Laid-Open No. 8-213645 discloses separation of thin films of monocrystalline Si, making use of epitaxial layers on a porous Si layer. FIG. 16 is a cross-sectional view showing a method of forming a thin-film Si solar cell, described in Japanese Patent Application Laid-Open No. 8-213645. In the figure, numeral 101 designates an Si wafer, 102 a porous Si layer, 103 a p+ type Si layer, 104 a pxe2x88x92 type Si layer, 105 an n+ type Si layer, 106 a protective film, 109 and 111 adhesive layers, and 110 and 112 jigs. In the process of producing the solar cell of FIG. 16, the porous Si layer 102 is made by anodization over the surface of Si wafer 101. After that, the p+ type Si layer 103 is epitaxially grown on the porous Si layer 102, and the pxe2x88x92 type Si layer 104 and n+ type Si layer 105 are further grown thereon. Then, the protective film 106 is formed thereon. The adhesive layers 111, 109 are then laid over the protective film 106 and over the Si wafer 101, respectively, to be bonded to the jigs 112, 110. Thereafter, pulling forces P are exerted on the respective jigs 110, 112 to separate the Si wafer 101 from the epitaxially grown Si layers (103, 104, 105) across the porous Si layer 102. Then, the solar cell is formed in the epitaxially grown Si layers (103, 104, 105), while the Si wafer 101 is again subjected to similar steps, thereby reducing the cost.
Japanese Patent Application Laid-Open No. 5-283722 discloses growth of epitaxial Si layers on the porous Si layer by the liquid phase growth method. An Sn melt is used as a solvent, and Si is preliminarily dissolved into the Sn melt to saturate therein, prior to the growth. Then, the melt is slowly cooled, and at a certain level of supersaturation a porous surface of a wafer is dipped into the Sn melt to grow an epitaxial Si layer on the porous surface.
Japanese Patent Application Laid-Open No. 5-17284 discloses an immersion type liquid phase growth apparatus of compound semiconductor and a holding jig. FIG. 17 is a cross-sectional view of this liquid phase growth apparatus. In the figure, numeral 81 designates a wafer holder, 82 a wafer, 83 a crucible, 84 a solution, 85 a quartz reactor tube, 886 a gas introducing tube, 887 a gas exhaust tube, 88 a heater, and 89 a dummy wafer. In this immersion type liquid phase growth apparatus, the wafer holder 81 holding the wafer 82 and dummy wafer 89 is moved down (in the direction A), the wafer 82 is immersed into the solution 84 in which a growth material is dissolved. The solution 84 is retained in the crucible 83 and the crucible 83 is placed in the quartz reactor tube 85 which maintains the interior in an atmospheric (or ambient) gas (reducing gas or inert gas) by means of the gas introducing tube 86 and gas exhaust tube 87. The heater 88 is provided for control of temperature of the system. The temperature of the solution 84 is lowered by decreasing the temperature of the heater 88, whereby the growth material is precipitated from the solution 84 onto the wafer 82 to grow in liquid phase. The immersion type liquid phase growth apparatus can be constructed in smaller size as the growth apparatus for liquid phase growth on wafers of the same size than the liquid phase growth apparatus of the slide boat type and the liquid injection type. The immersion type liquid phase growth apparatus is also convenient for mass production, because a lot of wafers can be set on the holder.
Japanese Patent Application Laid-Open No. 57-76821 also discloses an immersion type liquid phase growth method. FIG. 18A is a perspective view of a wafer holder disclosed in Japanese Patent Application Laid-Open No. 57-76821. Numeral 123 designates an arm, 124 an umbrella-like plate, 125 wafers, 126 a through hole, and 122 a cylindrical member. This wafer holder can be loaded with six substrates on the umbrella-like plate 124. FIG. 18B is a plan view of this wafer holder. FIG. 18C shows a crucible 128 into which the wafer holder shown in FIGS. 18A and 18B is immersed, and a solution 129 is filled in the crucible 128. A heater 130 is placed around the crucible 128 and an auxiliary heater 131 projecting upright is located in the center and in the lower part of the crucible 128. Namely, the crucible 128 is recessed toward the interior around the auxiliary heater 131 and the auxiliary heater 131 is placed inside the recess. Since this auxiliary heater 131 prevents the central part of the solution 129 from becoming lower in temperature than the peripheral part, the solution 129 is wholly kept at even growth temperature. It is disclosed that this arrangement can thus implement the uniform liquid phase growth.
In the case of the immersion type liquid phase growth apparatus disclosed in Japanese Patent Application Laid-Open No. 5-17284 described referring to FIG. 17, when the wafer size is large for liquid phase growth over a large area, a deposited film becomes thick in the peripheral part of the wafer but thin in the central part. FIG. 19 is a cross-sectional view showing the growth of the deposited film. In the figure, numeral 82 denotes a wafer and 90 an epitaxial film deposited on the wafer 82. As illustrated, for the large wafer size, the deposited film 90 becomes thin in the central part of the wafer but thick in the peripheral part. This phenomenon becomes noticeable, particularly, when the wafer size is not less than five inches. A conceivable reason for it is that the outside part of the solution 84 is close enough to the atmospheric gas to reduce the temperature of the outside part of the solution according to the cooling of the system whereas the central part of the solution 84 is far from the atmospheric gas and is thus cooled with a lag behind the temperature reduction of the outside part. Namely, a temperature change is less since a growth start in the central part of the wafer than in the peripheral part. For this reason, a deposition amount of the deposited film 90 is considered to be smaller in the central part of the wafer than in the peripheral part.
This is also the case in the liquid phase growth method disclosed in Japanese Patent Application Laid-Open No. 57-76821, and the temperature is less unlikely to be reduced in the central part of the crucible than in the peripheral part, because there is no mechanism of intentionally cooling the solution in the crucible. For this reason, a precipitation amount of the solute is less in the central part of the crucible, so that the thickness of the deposited film is also thinner on the wafer near the central part of the crucible.
When the deposited film on the wafer has dispersion of thickness as described above, there is a possibility of contact failure, for example, when electrodes are attached to the surface of the deposited film in production of solar cell. In addition, since the thickness of the deposited film has to be determined in the thin part for sufficient absorption of light, the thick part becomes over specifications and thus the material of the deposited film is wasted. Besides, where the deposited film is thick, the peripheral part of the film is too thick for photocarriers to reach the electrodes because of the too large thickness, which degrades light conversion efficiency. For this reason, when the solar cell is made on the wafer, the deposited film is preferably made as uniform as possible. In the case of semiconductor devices other than the solar cells, the dispersion of thickness of the deposited film will require different design between in the central part and in the peripheral part of the wafer, which will increase the number of steps and, in turn, increase the production cost.
An object of the present invention is to solve the above problems separately or all together. Specifically, an object of the invention is to form a deposited film having a uniform thickness entirely over the substrate by the liquid phase growth on the substrate such as the wafer or the like.
In order to solve the aforementioned problems, the inventors conducted elaborate research and accomplished the following invention as a result.
For achieving the above object, an aspect of the present invention is a liquid phase growth method comprising the steps of:
immersing a substrate in a crucible that stores a solvent having a growth material dissolved therein; and
cooling the solvent from an interior thereof.
The solvent is preferably cooled from a central part thereof.
It is preferable to implement the cooling step by letting a medium flow through a tube immersed in the crucible; by letting a medium flow through a hole formed in a jig for holding the substrate; or by letting a medium flow through a hole formed in the crucible.
Another aspect of the present invention is a liquid phase growth apparatus comprising:
a crucible for storing a solvent having a growth material dissolved therein;
a wafer cassette for holding a substrate to be immersed in the solvent; and
a cooling section for cooling the solvent from an interior thereof.
The cooling section is preferably a tube which is immersed in the crucible and through which a medium is made to flow, a hole which is formed inside a wafer cassette and through which a medium is made to flow, or a hole which is formed in the crucible and through which a medium is made to flow.
The foregoing medium is, for example, a gas. The gas is preferably hydrogen or nitrogen gas as an atmospheric gas.
A liquid phase growth bath for formation of a pxe2x88x92 type Si layer may be used as the crucible and, subsequent thereto, a liquid phase growth bath for formation of an n+ type Si layer may be used as the crucible.
The growth material is, for example, Si, Ge, or GaAs.
The solvent is, for example, a melt of In or Sn.
The wafer cassette is preferably rotatable about its own axis.
The wafer cassette is preferably revolvable about an axis different from its own axis.